1. Field of the Invention
The present invention relates generally to analog integrated circuits. More specifically, a low power, high performance circuit for magnitude and group delay shaping in continuous-time read channel filters is disclosed.
2. Description of Related Art
High-frequency continuous-time filters are primarily used in mixed-signal integrated circuits for anti-aliasing and reconstruction functions. In hard disk drive applications, continuous-time filters, also referred to as read channel filters, provide channel equalization via magnitude and group delay shaping. Equalization in analog domain is advantageous in terms of power, die size, reduced clock latency and optimized dynamic range for the analog-to-digital converter, while equalization in digital domain offers programmability and architectural flexibility. Current state of the art read channel filters realize seventh order linear phase responses to provide up to 13 dB of magnitude boost, i.e., increase in magnitude without a change in phase, and approximately 30% group delay shaping for a maximum unboosted filter cut-off frequency F.sub.c of about 70 MHz to support data rates of up to 250 Mbps.
Filter magnitude and group delay shaping are important functions in continuous time filters, such as in read signal equalization, video signal conditioning, cable equalizers, etc. Specifically, in disk-drive applications, the readback signal has traditionally been pulse slimmed in time domain in order to mitigate the undesirable effects of intersymbol interference, a phenomenon that occurs due to close spacing of adjacent bits. FIG. 1 shows a readback signal with and without pulse slimming in the time domain. Pulse slimming is generally equivalent to accentuating the mid-to-high frequency region of the read pulse spectrum, or a magnitude boost in an appropriate frequency band. Pulse slimming should not distort the group delay and hence the boost is also referred to as symmetric boost.
In the frequency domain, pulse slimming is equivalent to shaping the magnitude as a function of frequency. FIG. 2 shows a readback signal with and without pulse slimming in the frequency domain. Specifically, curve 20 represents a traditional unboosted low pass response and curve 22 represents a boosted magnitude which is well defined as a function of frequency.
Although the problem of intersymbol interference ("ISI") has been exacerbated in high density and high speed disk-drives, the availability of powerful digital signal processing ("DSP") techniques enable the detection of readback signals reliably and efficiently. However, this detection process typically demands increasing amounts of boost, exceeding 13 dB relative to the unboosted value at the lowpass corner frequency.
In addition to magnitude shaping, the phase response or group delay of the readback signal is optionally conditioned to compensate for the nonlinear phase responses, of circuits, caused by finite bandwidth and DC offset-cancellation needs. Group delay shaping, also referred to as asymmetric boost, is different from magnitude shaping or symmetric boost. FIG. 3 shows effects of modifying the group delay as a function of frequency when compared to a nominally flat group delay of the read channel filter. Specifically, curve 30 shows a nominally flat group delay of the read channel filter while curves 32, 34, 36, and 38 show the effects of modifying the group delay to various extents as a function of frequency. As is evident from the graph of FIG. 3, with data rates increasing to 1 Gbps, as much as 40% of the group delay compensation may be required.
Increases in the hard disk drive data rates increase the user bit density, which in turn imposes more stringent requirements on magnitude and group delay equalization. Thus, a circuit that is both power and die-area efficient to achieve magnitude and group delay shaping for high data rate read channel filters is highly desired.